1. Field of the Invention
The present invention relates to the design and manufacture of double casing pipes, such as in particular pipes from which pipelines are assembled by joining sections end to end, or those which are more specifically built and assembled into pipelines to be laid on the bottom of the oceans, for the purpose of transporting oil products, in the liquid or gaseous state.
For this or other similar industrial applications, the invention provides improvements in pipes which are generally equipped with a heat insulating double casing so as to provide a sealed annular space between two coaxial tubes.
2. Description of the Related Art
One recurrent problem involved in the manufacture of such pipes arises from the need to achieve good heat insulation between the inner portion and outside environment of the pipe, at least at certain periods during the service lifetime of a pipeline assembled therefrom. By way of illustration, submarine pipeline used to transport oil products are as a matter of fact subjected to cold temperatures at the seabed level (usually between 0.degree. and 20.degree. C.), whereas the fluids being carried need often be at relatively high temperatures (in the range of 100.degree. to 200.degree. C. according to current specifications). Even though temperature may decrease for a production field in the course of time, the fluid temperature must not drop below a minimum value (say 40.degree. C.) before it reaches its final delivery site located at the end of the pipeline, in order to prevent the formation of solid deposits.
In addition, distances being covered are significantly high and are expressed in tens of kilometers. It is essential that the heat insulation capacity last as so many years as the pipeline is in service. Among other requisites from a practical standpoint, it might be stressed that heat insulation capacity should not be adversely affected by the assembling operations into pipelines during which submarine pipes are sequentially joined end to end, nor during the laying process of the resulting pipeline, which is gradually lowered into the sea, nor even from transport between a pipe production line or facility and the site where the pipeline is actually installed.
Other limitations arise from the process of pipe production, inevitable dimensional tolerances which are inherent to tubes, depending on their nature (typically steel) and their diameter (generally of between 100 and 700 mm), and to sealing treatments. One proposal considered by the oil industry to circumvent the above problem resides in generating high vacuum within the annular space located between the two coaxial tubes of a double casing pipe. In this case, it is evident that a good seal is critical to achieve and that both the required vacuum and the thickness compatible therewith result in the pipe cost being undesirably increased.
Similar heat insulation and durability requirements hold true for other applications, notably for conditions where a temperature differential of the same range is otherwise found between the inner and the outer portion of the pipe along the opposite direction.